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Downplayed: Eva transforms into Bob's pet dog, which makes Bob confused.
Eva knows about Bob's past to hit where it hurts.
Eva feeds off misery to live, and her time is nearly up so she takes Alice's form to feed off Bob's emotions.
Eva isn't well adapted/trained for direct confrontations, with her shapeshifting abilities being better suited for stealth and infiltration missions. So, when she ends up engaged in a duel with Bob, she decides to take Alice's form to confuse him and try to make a shift escape.
Eva transforms into Bob's sworn-enemy, and taunts him in order to make him lose his composure and judgment.
Eva is on Bob's side, and transforms into Bob's mentor to give him more confidence.
Eva transforms into Alice, but Bob gives no response...
Alternatively, all the transformation manages to do is piss Bob off. "You don't deserve to wear that face!"
Double Subverted: Until Eva, as Alice, calls him out for letting her die. Cue BSOD.